Catalytic reforming involves a number of competing processes or reaction sequences. These include dehydrogenation of cyclohexanes to aromatics, dehydroisomerization of alkylcyclopentanes to aromatics, dehydrocyclization of an acyclic hydrocarbon to aromatics, hydrocracking of paraffins to light products boiling outside the gasoline range, dealkylation of alkylbenzenes and isomerization of paraffins. Some of the reactions occurring during reforming, such as hydrocracking which produces light paraffin gases, have a deleterious effect on the yield of products boiling in the gasoline range. Process improvements in catalytic reforming thus are targeted toward enhancing those reactions effecting a higher yield of the gasoline fraction at a given octane number.
It is of critical importance that a catalyst exhibits the capability both to initially perform its specified functions efficiently and to perform them satisfactorily for prolonged periods of time. The parameters used in the art to measure how well a particular catalyst performs its intended function in a particular hydrocarbon reaction environment are activity, selectivity and stability. In a reforming environment, these parameters are defined as follows:
(1) Activity is a measure of the ability of the catalyst to convert hydrocarbon reactants to products at a designated severity level, with severity level representing a combination of reaction conditions: temperature, pressure, contact time, and hydrogen partial pressure. Activity typically is characterized as the octane number of the pentanes and heavier (“C5+”) product stream from a given feedstock at a given severity level, or conversely as the temperature required to achieve a given octane number.
(2) Selectivity refers to the percentage yield of petrochemical aromatics or C5+ gasoline product from a given feedstock at a particular activity level.
(3) Stability refers to the rate of change of activity or selectivity per unit of time or of feedstock processed. Activity stability generally is measured as the rate of change of operating temperature per unit of time or of feedstock to achieve a given C5+ product octane, with a lower rate of temperature change corresponding to better activity stability, since catalytic reforming units typically operate at relatively constant product octane. Selectivity stability is measured as the rate of decrease of C5+ product or aromatics yield per unit of time or of feedstock.
Programs to improve performance of reforming catalysts are being stimulated by the reformulation of gasoline, following upon widespread removal of lead antiknock additive, in order to reduce harmful vehicle emissions. Gasoline-upgrading processes such as catalytic reforming must operate at higher efficiency with greater flexibility in order to meet these changing requirements. Catalyst selectivity is becoming ever more important to tailor gasoline components to these needs while avoiding losses to lower-value products. The major problem facing workers in this area of the art, therefore, is to develop more selective catalysts while maintaining effective catalyst activity and stability.
Reforming catalysts containing tin as platinum-group (or Group VIII) a modifiers, along with optional third metal promoters such as rhenium, indium, gallium, iridium, etc., are well known in the art. For example, U.S. Pat. No. 3,830,727 discloses a process for catalytic reforming using a catalyst comprising a platinum, rhenium, and tin, along with a halogen and a halogen activation step. This catalyst is prepared by impregnating the support with the desired components. U.S. Pat. No. 6,153,090 discloses a process for catalytic reforming using a catalyst comprising at least one group VIII metal, at least one additional element selected from the group consisting of germanium, tin, lead, rhenium, gallium, indium, thallium, where the promoter element is added in the form of an organometallic carboxylate compound containing at least one organometallic bond such as tributyl tin acetate.
It is also known that chelating ligands can be used to impregnate metals onto a support. For example, U.S. Pat. No. 4,719,196 discloses preparing a catalyst using a solution containing ethylene diaminetetraacetic acid (EDTA), a noble metal and ammonia U.S. Pat. No. 5,482,910, which is incorporated herein by reference thereto, discloses a process for preparing a catalyst using a mixed solution comprising EDTA, a noble metal, and a promoter metal, such as an alkali earth metal. U.S. Pat. Nos. 6,015,485 and 6,291,394 disclose a process for treating an existing catalyst with EDTA in order to create a bimodal mesopore structure with alumina at two different crystallite sizes. No references to applicants' knowledge disclose the use of EDTA or a related chelating agent to impregnate tin onto a catalyst support.
Accordingly, applicants have developed a process for preparing catalysts which involves the use of a tin chelate complex to impregnate the tin component. The process involves preparing a tin solution containing a chelating ligand such as EDTA. This solution is heated and then used to impregnate a refractory oxide support such as alumina. Before or after the chelated impregnation, another solution can be used to impregnate platinum-group metals and any other desired promoter metals such as rhenium. Preferably, the impregnation with the tin chelate is performed at basic conditions, while the impregnation of the other components is performed at acidic conditions. After impregnation, calcination and reduction provide the desired catalyst.